In the manufacture of molded records, it is conventional to mold a thermoplastic resin composition between a set of metal stampers to form the desired record. The metal record stampers have formed in their molding surfaces the information desired to be molded into the surface of the record.
The stampers are the final product of a replication process referred to as matrixing. The initial steps in the matrixing process is to cut the desired recording into a suitable substrate that can thereafter be replicated. Recordings for audio records are generally cut into wax or lacquer substrates. Recording for records such as video disc, and the like, where the signal elements desired to be recorded are considerably smaller than for audio records, are cut in a much harder substrate such as an electroformed copper substrate.
After the desired recording is made in the substrate, the substrate is then replicated by electroforming a metal, preferably nickel, onto the surface of the substrate. The electroformed part which is obtained is referred to as a master. The master is then replicated a number of times to produce a series of parts which are referred to as molds. The molds in turn are then replicated by electroforming a metal, preferably nickel, onto the surface of the molds to form stampers, which are negative copies of the original recorded substrate.
The stampers, after they are separated from the molds, are trimmed and shaped. The stampers are then mounted on the molding platens of a record molding press. In the operation of the press, a thermoplastic molding composition is placed between a pair of the stampers and the stampers are pressed together to form the molded record. The quality and fidelity of the molded record to the original recording is directly related to the quality of the stamper with which it is pressed.
There are a number of important properties which are required in stampers in order for the stampers to be satisfactory for the molding of records, especially in commercial production. The most important property is that the molding surface of the stamper must have sufficient release property so that the molding composition does not stick to the surface of the stamper. If the molding composition does adhere to the surface of the stamper a number of problems are encountered. Even slight sticking of the molding composition to the stamper results in distortion of the recorded signal element and as a result produces defective records. Furthermore, if the molding composition adheres strongly to the surface of the stamper, when the press is opened it can often cause distortion and destruction of the stampers as they are pulled apart by the opening action of the press.
In addition to having non-sticking properties, the surface of the stamper must likewise be scratch resistant, as any scratches on the surface of the stamper result in undesired noise in the molded record. Scratching of the stampers is a long-standing problem in the manufacture of molded records, and is especially troublesome in the manufacture of a video disc, and the like, because of the small size of the recorded signal elements in comparison to even microsize scratches. Scratching problems are especially troublesome in the manufacture of video discs because the compositions which are used in the manufacture of the video disc, contain substantial amounts of conductive carbon particles which can cause microscratching of the surface of the stampers.
A further property which is required in the stampers is that they be resistant to chemical attack. If the surfaces of the stamper are not relatively chemically inert, additives conventionally used in the molding compounds attack the surface of the stamper and cause staining of the surface of the stamper. The problem of staining is especially severe in the case of compositions used for the molding of video discs, and the like, because these compounds inherently must contain considerably greater amounts of various additives, such as mold releases, lubricants and the like.
Untreated virgin nickel stampers, as formed in the matrixing processes, have been found to be generally unsuitable for use in the molding of records on a commercial scale. The plastic compositions used for the molding of records stick excessively to virgin nickel surfaces unless substantial amounts of mold releases are added to the compositions. The addition of the mold release compounds on the other hand causes a further undesirable effect of promoting development of stains on the surface of the stampers. Untreated nickel stampers also have a relatively soft surface so that they can be easily scratched. The virgin nickel surface of the stampers is likewise quite chemically active and numerous reactions occur between the nickel stampers and the molding compositions which can cause deterioration of the quality of the molding surface of the stamper.
The method which has heretofore been widely used to provide additional protection for metal stampers, and especially for nickel stampers, is to electroplate the surface of the stampers with chromium. The chromium appears to give some additional protection to the surface of the stampers from mechanical damage, such as scratches, and also appears to passivate the surface of the stampers.
Chrome plating, however, is not a completely satisfactory solution. The amount of protection provided to the stampers by the chrome plating appears to be directly related to the relative thickness of the chrome plating applied to the surface of the stampers. Increasing the thickness of the chrome plating however, results in a corresponding decrease in the quality of the records molded from the stamper in that the chrome plating tends to fill in and obliterate the recorded signal elements in the grooves of the stamper. With conventional audio records there has generally been an accepted tradeoff of fidelity of the final recorded records in order to obtain the protection afforded by chrome plating. However, with records having extremely small recorded features, such as the video disc, it was found that the stampers could not effectively be chrome plated in that chroming virtually obliterated the recorded signal information.
A further problem of chrome plating is that even on audio stampers, and the like, chrome plating is at best a temporary to a semi-permanent treatment. On extended press runs, the chrome plating is gradually removed from the surface of the stampers. As the chrome plating is removed, there is a tendency for sticking and chemical reactions to occur at the stamper surfaces which eventually cause failure of the stampers, especially on extended press runs.
A further related problem of chrome plating is that the plating solutions that are used in the plating process, once they are spent, are extremely difficult to dispose of because of environmental pollution problems.
An additional problem encountered with chrome plating stampers, is that the chrome, when applied to the stampers, substantially reduces the scrap value of the nickel in the stampers as compared to pure nickel scrap. The reduction in cost is quite substantial, and results in increasing the overall production cost.
It would be highly advantageous if a method could be provided to treat the molding surface of metal stampers, especially nickel stampers, to make them more scratch resistant and chemically passive but which would not have the inherent disadvantage of chrome plating.